You say your "rear patio door opens directly over the tiles" Do you mean paving slabs or tiles on a roof?
As you have found it is both a Building Regulation and NHBC standard requirement that DPCs are provided in external walls at a minimum 150mm above finished ground level.
However, in order to comply with Part M (Mobility) Building Regulations, the maximum step up from an external door for mobility is 15mm.
As the DPC is level and under your door, in this case it be less than 150mm (at the door only) and the ground level should slope downwards, or the DPC stepped down, either side of the door until the 150mm is achieved.
The 150mm requirement is designed to stop rain splashing up off the ground above the DPC and causing damp by soaking a wall. As your patio door is presumably impervious, this should not matter.
The requirement reference alternative in Part C of the Building Regulations you mention applies to walls subject to ground water pressure and refers to tanking details etc.
Nevertheless the 150mm above finished ground level requirement is still necessary for the external wall DPC in this case.